Field of the Invention
The invention relates in general to multimedia signal processing technologies, and more particularly to encoding/decoding technologies in a video system.
Description of the Related Art
Digital television broadcasting has matured and become popular with the ever-improving communication technologies. In addition to being transmitted through cables, digital television signals may be propagated in form of wireless signals via base stations or artificial satellites. To satisfy demands on enhanced image quality and reduced transmission data amount, a transmitter end usually encodes and decompresses audio/video signals to be transmitted. Correspondingly, a receiver end needs to correctly decode and decompress the received signals in order to restore the audio/video signals.
FIG. 1 shows a partial functional block diagram of an encoding system compliant to the digital audio video coding standard (AVS). An intra-prediction module 12 performs an intra-prediction process on image blocks in a video frame to generate luminance residual blocks corresponding to the image blocks. For example, FIG. 2(A) to FIG. 2(D) depict several different reference modes considered when the intra-prediction module 12 performs intra-prediction. An image block 20 includes 8*8 pixels. In the reference mode in FIG. 2(A), the intra-prediction module 12 regards luminance values of eight reference points (the shaded points) at the top of the image block 20 as reference values to calculate luminance residual values of the pixels in the image block 20. As shown by the arrow pointing vertically downward in FIG. 2(A), the pixels in each column correspond to the reference point right above, and the luminance residual value of each pixel is a difference between its luminance value and the luminance value of the corresponding reference point. Thus, the size of the luminance residual block is same as that of the image block 20, i.e., including 8*8 luminance residual values.
Different reference modes adopt different reference points. In the reference mode in FIG. 2(B), the intra-prediction module 12 regards luminance values of eight reference points at the left of the image block 20 as reference values. In the reference mode in FIG. 2(C), the intra-prediction module 12 regards luminance values of 15 reference points at the upper right of the image block 20 as reference values. In the reference mode in FIG. 2(D), the intra-prediction module 12 regards luminance values of 15 reference points at the top and left of the image block 20 as reference values. Details of intra-prediction and other reference modes may be referred from technical documents provided by the AVS work team. In general, contents of luminance residual blocks obtained from different reference modes are different. The intra-prediction module 12 estimates data amounts and distortion rates of converted and quantized luminance residual blocks corresponding to the reference modes, selects a luminance residual block that most satisfies two conditions including a small data amount and a low distortion rate therefrom, and regards the selected the luminance residual block as the luminance residual block representing the image block 20.
The luminance residual block selected by the intra-prediction module 12 is provided to a discrete cosine transform (DCT) module 14 for a DCT process to generate a DCT coefficient matrix. In this example, the size of the DCT coefficient matrix, same as that of the luminance residual block, includes 8*8 DCT coefficients. To further reduce the data amount, a secondary transform module 16 performs a secondary transform process on low-frequency components in the DCT coefficient matrix. According to the AVS specification, regardless of the size (N*N) of the DCT coefficient matrix, the secondary transform module 16 performs the secondary process on only 4*4 low-frequency components at the upper left (as shown in FIG. 3). Similar to the DCT process, the secondary transform process actually sequentially includes secondary transform performed along a vertical direction and secondary transform performed along a horizontal direction. The low-frequency components having undergone the secondary transform and other high-frequency DCT coefficients that have not been processed by the secondary transform are recombined at a quantization module 18, which then performs a quantization process on the DCT coefficients.
Known to one person skilled in the art, spatial correlation is usually present among adjacent pixels of a same image. That is to say, image data of two adjacent pixels does not differ drastically under most circumstances. The foundation of the secondary transform performed after the DCT process is based on the assumption of the presence of spatial correlation. More specifically, when spatial correlation is present between the image block and the reference points at the top of the image block, performing secondary transform along the vertical direction on the low-frequency components in the DCT coefficient matrix reduces the data amount. Similarly, when spatial correlation is present between the image block and the reference points at the left of the image block, performing secondary transform along the horizontal direction on the low-frequency components in the DCT coefficient matrix also reduces the data amount. However, one issue of current solutions is that, whether spatial correlation is present between an image block and its reference points when the secondary transform module 16 performs secondary transform is not considered. Thus, the secondary transform performed by the secondary transform module 16 is sometimes unnecessary, and may even yield an undesired effect of an increased data amount after the transform process.